Humidity under basking lamps

These graphs illustrate the dramatic reduction in ambient humidity experienced directly beneath various common reptile basking lamps. In all cases
humidity is reduced very rapidly after the lamp is turned on. Typical room ambient humidity levels were employed as a starting position. The general
room ambient was also monitored simultaneously at a distance of 2M using an independent data logger.

In this first graph ambient room humidity level was 70%. Within seconds, under the basking lamp the levels fell to 30%, then to below 20%.

The lamp was switched off and levels recovered.

It was then turned on again and levels fell as before.  Levels of 20% and below are extremely damaging if sustained, even for arid habitat species.

On-off test

These are the effects of a 60w basking lamp (as frequently used with hatchlings) at 30cm above the substrate.

The first is measured directly under the lamp, in the centre. Starting humidity (background ambient) was 41%. The lamp was tuned on at 11.00 am and off at 1.20 pm.

60w lamp test 1

The minimum RH attained was 14.83%

The second graph shows the effect at 10cm from the edge of the lamp, also at substrate level. Times are identical as both recordings were simultaneous.

60w lamp test 2

The minimum RH attained was 19%.

Both of these levels are extremely low and sustained levels like this would without doubt contribute to a state of chronic dehydration and high rates of fluid loss, even in a 'desert' reptile.

One consequence. Bladder 'stones' as seen here. History: tortoise purchased from pet shop and maintained in terrarium with overhead 100w basking lamp. Fresh water available at all times.

Bladder stone in tortoise

For comparison, this is what actually happens in the wild, during the very hottest and driest part of the year.
Measured this last August (the hottest, driest August on record for 100 years with T. graeca graeca).
These measurements were obtained from an attached logger in direct contact with an estivating tortoise.

Estivation microclimate